Drug Interactions Practice Questions

Q: Which medication should be avoided in a patient with asthma who is currently on theophylline therapy, prior to dental extraction?

Ciprofloxacin. **Explanation:** The correct answer is **Ciprofloxacin**. This question tests the knowledge of **Cytochrome P450 (CYP450) enzyme inhibition** and its impact on drugs with a narrow therapeutic index, like Theophylline. **1. Why Ciprofloxacin is correct:** Theophylline is primarily metabolized by the hepatic enzyme **CYP1A2**. Ciprofloxacin is a potent inhibitor of CYP1A2. When co-administered, Ciprofloxacin inhibits the metabolism of Theophylline, leading to significantly elevated serum levels. Since Theophylline has a **narrow therapeutic index**, this interaction can result in toxicity, manifesting as severe nausea, vomiting, cardiac arrhythmias, and seizures. **2. Why the other options are incorrect:** * **Lignocaine:** While Lignocaine is also metabolized by CYP enzymes, it does not significantly inhibit the metabolism of Theophylline. It is the standard local anesthetic used in dental extractions. * **Azithromycin:** Unlike other macrolides (such as Erythromycin or Clarithromycin), Azithromycin does **not** significantly inhibit CYP450 enzymes and is generally considered safe to use with Theophylline. * **Metronidazole:** This drug primarily inhibits CYP2C9 (affecting Warfarin). It does not have a clinically significant interaction with Theophylline metabolism. **Clinical Pearls for NEET-PG:** * **Theophylline Toxicity:** Remember the "Three C's" of enzyme inhibitors that increase Theophylline levels: **C**iprofloxacin, **C**imetidine, and **C**larithromycin/Erythromycin. * **Narrow Therapeutic Index Drugs:** Always monitor for interactions when a patient is on Theophylline, Warfarin, Digoxin, or Lithium. * **Safe Alternative:** If an antibiotic is needed for a patient on Theophylline, **Amoxicillin** or **Azithromycin** are preferred choices.

Q: Which of the following is NOT an enzyme inhibitor?

Carbamazepine. **Explanation:** The core concept tested here is the distinction between **Microsomal Enzyme Inducers** and **Inhibitors**. **1. Why Carbamazepine is the correct answer:** Carbamazepine is a potent **Enzyme Inducer**, not an inhibitor. It increases the synthesis of Cytochrome P450 (CYP) enzymes in the liver. A unique clinical feature of Carbamazepine is **auto-induction**, meaning it induces its own metabolism over the first few weeks of therapy, necessitating dosage adjustments. Because it induces enzymes, it decreases the plasma concentration and efficacy of co-administered drugs like Warfarin or Oral Contraceptive Pills. **2. Analysis of Incorrect Options:** * **Cimetidine:** A classic, broad-spectrum enzyme inhibitor. It frequently causes drug-drug interactions by increasing the levels of drugs like Theophylline and Phenytoin. * **Valproate:** Unlike most anti-epileptics (which are inducers), Valproate is a significant enzyme inhibitor. It commonly increases the levels of Phenobarbital and Lamotrigine, increasing the risk of toxicity. * **Risperidone:** While primarily an antipsychotic, it acts as an inhibitor of the CYP2D6 isoenzyme. **3. NEET-PG High-Yield Pearls:** To quickly differentiate these for the exam, remember these mnemonics: * **Enzyme Inducers (GPRS Cell Phone):** **G**riseofulvin, **P**henytoin, **R**ifampicin, **S**moking, **C**arbamazepine, **P**henobarbitone. * **Enzyme Inhibitors (VITAMIN K):** **V**alproate, **I**soniazid, **T**urmeric (Curcumin), **A**miodarone, **M**acrolides (except Azithromycin), **I**traconazole, **N**ilotinib, **K**etoconazole (and Cimetidine/Grapefruit juice). **Key Takeaway:** Carbamazepine is an inducer; Cimetidine and Valproate are inhibitors. Risperidone has inhibitory effects on specific CYP pathways.

Q: A patient with temporal lobe epilepsy (complex partial seizures) is experiencing recurrent seizures despite being on carbamazepine. Phenobarbital is added as a second drug, but the seizures increase in frequency. What is the probable reason for the apparent adverse effect of adding phenobarbital?

Decreased carbamazepine level. ### Explanation **1. Why the Correct Answer is Right:** The primary mechanism behind this interaction is **Enzyme Induction**. Both Carbamazepine and Phenobarbital are potent inducers of the hepatic cytochrome P450 system (specifically **CYP3A4**). When Phenobarbital is added to a stable regimen of Carbamazepine, it accelerates the metabolism of Carbamazepine. This leads to a significant **decrease in the plasma concentration** of Carbamazepine, falling below the therapeutic window and resulting in a loss of seizure control (breakthrough seizures). **2. Why the Incorrect Options are Wrong:** * **Option A:** While Carbamazepine can cause bone marrow suppression (aplastic anemia), this typically leads to infections or bleeding diathesis, not an immediate increase in seizure frequency. * **Option C:** Both drugs are anticonvulsants that *increase* the stability of neuronal membranes (Carbamazepine via sodium channel blockade; Phenobarbital via GABA-A receptor facilitation). They do not destabilize membranes. * **Option D:** Hypokalemia is not a recognized side effect of these anticonvulsants. Carbamazepine is more famously associated with **hyponatremia** (SIADH-like effect). **3. Clinical Pearls for NEET-PG:** * **Auto-induction:** Carbamazepine is unique because it induces its own metabolism (*auto-induction*), meaning doses often need adjustment after the first few weeks of therapy. * **Enzyme Inducers (Mnemonic: GPRS Cell Phone):** **G**riseofulvin, **P**henytoin, **R**ifampicin, **S**moking, **C**arbamazepine, **P**henobarbital. * **Drug of Choice:** Carbamazepine remains a first-line agent for Focal (Partial) seizures and Trigeminal Neuralgia. * **Therapeutic Drug Monitoring (TDM):** This scenario highlights the importance of TDM when combining multiple anti-epileptic drugs (AEDs) to manage drug-drug interactions.

Q: The level of drug metabolizing enzymes in the liver determines how fast a drug is removed from the circulation. Therefore, it would be expected to find drug metabolizing enzymes:

Higher in smokers than in nonsmokers. **Explanation:** The correct answer is **A. Higher in smokers than in nonsmokers.** This question tests the concept of **Enzyme Induction**. Cigarette smoke contains **Polycyclic Aromatic Hydrocarbons (PAHs)**, which act as potent inducers of hepatic microsomal enzymes, specifically the **CYP1A1, CYP1A2, and CYP2E1** isoforms. 1. **Why A is correct:** Enzyme induction involves the increased synthesis of cytochrome P450 enzymes. In chronic smokers, the liver produces a higher concentration of these enzymes to metabolize the toxins in smoke. Consequently, other drugs metabolized by the same pathways (e.g., Theophylline, Clozapine, Warfarin) are cleared faster, often requiring higher doses to achieve therapeutic effects. 2. **Why B and C are incorrect:** Smoking is a well-documented environmental factor that alters pharmacokinetics. It does not leave enzyme levels unchanged (B), nor does it inhibit them (C). 3. **Why D is incorrect:** Malnutrition generally **decreases** drug-metabolizing enzyme levels due to a deficiency in the amino acids and cofactors (like iron and vitamins) necessary for enzyme synthesis and function. **High-Yield Clinical Pearls for NEET-PG:** * **Theophylline & Smoking:** This is a classic exam favorite. Smokers require higher doses of Theophylline because CYP1A2 induction increases its clearance. If a patient quits smoking, the dose must be reduced to avoid toxicity. * **Other Common Inducers (GPRS Cell Phone):** **G**riseofulvin, **P**henytoin, **R**ifampicin, **S**moking, **C**arbamazepine, **P**henobarbitone. * **Enzyme Inhibition vs. Induction:** Induction (like smoking) takes 1–2 weeks to manifest as it requires new protein synthesis, whereas inhibition occurs almost immediately.

Q: Which of the following drugs shows a disulfiram-like reaction with alcohol?

Griseofulvin. **Explanation:** **1. Why Griseofulvin is Correct:** Griseofulvin is an antifungal agent used for dermatophytosis. It is well-known to cause a **disulfiram-like reaction** when consumed with alcohol. This occurs because the drug interferes with the enzyme **aldehyde dehydrogenase**, leading to the accumulation of acetaldehyde in the blood. Clinical symptoms include flushing, tachycardia, palpitations, nausea, and hypotension. **2. Why the Other Options are Incorrect:** * **Amphotericin B:** This is a polyene antifungal known for its significant nephrotoxicity and infusion-related reactions ("shake and bake" chills/fever), but it does not interact with alcohol via the disulfiram pathway. * **Terbinafine:** An allylamine used for onychomycosis. Its primary side effects are gastrointestinal upset and hepatotoxicity; it does not cause disulfiram-like reactions. * **Rifampicin:** An antitubercular drug and a potent **microsomal enzyme inducer**. While it is hepatotoxic, it does not inhibit aldehyde dehydrogenase. **3. NEET-PG High-Yield Pearls:** To excel in NEET-PG, remember the mnemonic **"PM G C"** for common drugs causing disulfiram-like reactions: * **P:** Procarbazine * **M:** Metronidazole (Most common MCQ answer), Moxalactam * **G:** **Griseofulvin** * **C:** Cefoperazone, Cefotetan, Chlorpropamide (1st gen Sulfonylurea) **Additional Fact:** Griseofulvin is a **microsomal enzyme inducer** (unlike most antifungals which are inhibitors) and its absorption is significantly increased when taken with a **fatty meal**.

Q: Which of the following drugs should be avoided with warfarin?

NSAIDs like Ibuprofen. **Explanation:** The interaction between **Warfarin and NSAIDs (like Ibuprofen)** is a high-yield clinical scenario due to the significantly increased risk of life-threatening hemorrhage. This interaction is **pharmacodynamic** in nature: 1. **Antiplatelet Effect:** NSAIDs inhibit COX-1, leading to decreased Thromboxane A2 synthesis, which impairs platelet aggregation. 2. **Mucosal Injury:** NSAIDs inhibit protective prostaglandins in the gastric mucosa, increasing the risk of GI ulceration and bleeding. When combined with Warfarin’s anticoagulant effect (inhibition of Vitamin K epoxide reductase), the body’s primary and secondary hemostatic mechanisms are both compromised. **Analysis of Incorrect Options:** * **Antacids (A):** Generally do not have a clinically significant interaction with warfarin absorption or metabolism. * **Benzodiazepines (B):** These are considered safe to use with warfarin as they do not induce or inhibit the CYP450 enzymes responsible for warfarin metabolism (primarily CYP2C9). * **Codeine/Paracetamol (D):** These are the preferred analgesics for patients on anticoagulants. While high-dose, chronic paracetamol can occasionally elevate INR, it is significantly safer than NSAIDs. **NEET-PG High-Yield Pearls:** * **CYP2C9 Inhibitors:** Drugs like **Amiodarone, Metronidazole, and Erythromycin** increase warfarin levels (elevated INR). * **CYP Enzyme Inducers:** **Rifampicin, Phenytoin, and Carbamazepine** decrease warfarin efficacy (low INR). * **Monitoring:** Warfarin therapy is monitored using **PT/INR** (Target: 2.0–3.0). * **Antidote:** For immediate reversal, use **Prothrombin Complex Concentrate (PCC)** or Fresh Frozen Plasma; for non-emergent reversal, use **Vitamin K1**.

Q: Concurrent administration of tetracyclines is most likely to impair the action of which of the following drugs?

Penicillin. ### Explanation **The Core Concept: Bacteriostatic vs. Bactericidal Interference** The correct answer is **Penicillin** because of a classic pharmacological antagonism. Penicillins are **bactericidal** agents that act by inhibiting cell wall synthesis. Crucially, they are most effective against bacteria that are actively multiplying and synthesizing new cell walls. **Tetracyclines**, on the other hand, are **bacteriostatic** drugs; they inhibit protein synthesis (30S subunit), which halts bacterial growth and multiplication. When administered concurrently, Tetracyclines stop the bacteria from dividing. Since the bacteria are no longer actively growing, the "target" for Penicillin (active cell wall synthesis) is removed, thereby significantly impairing Penicillin's efficacy. **Analysis of Incorrect Options:** * **A, B, and C (Lincomycin, Chloramphenicol, Erythromycin):** All three are primarily **bacteriostatic** protein synthesis inhibitors (acting on the 50S subunit). While combining different bacteriostatic drugs can sometimes lead to competition for binding sites (e.g., Macrolides and Chloramphenicol), they do not exhibit the same fundamental antagonism seen between a bacteriostatic and a bactericidal agent. **High-Yield NEET-PG Pearls:** * **The Rule of Thumb:** Avoid combining bacteriostatic drugs (Tetracyclines, Sulfonamides, Ethambutol) with bactericidal drugs (Penicillins, Cephalosporins, Aminoglycosides) unless specifically indicated (e.g., in TB or H. pylori treatment). * **Clinical Example:** This antagonism was historically proven in the treatment of pneumococcal meningitis, where the combination of Penicillin and Chlortetracycline resulted in higher mortality than Penicillin alone. * **Exception:** Aminoglycosides (bactericidal) are often combined with Beta-lactams (bactericidal) for **synergy** (e.g., in Enterococcal endocarditis).

Q: A woman who has been taking oral contraceptives for several years is diagnosed with epilepsy and started on phenytoin. Which of the following is the most likely consequence of adding phenytoin?

Reduced contraceptive efficacy. **Explanation:** The correct answer is **Reduced contraceptive efficacy**. This interaction is a classic example of **Cytochrome P450 (CYP450) enzyme induction**. **Mechanism:** Phenytoin is a potent inducer of hepatic microsomal enzymes, specifically the CYP3A4 isoenzyme. Oral contraceptive pills (OCPs) contain estrogen and progesterone, which are metabolized by these same enzymes. When phenytoin is added, it accelerates the metabolism of the hormones in the OCP, leading to sub-therapeutic plasma levels. This can result in breakthrough ovulation and unintended pregnancy. **Analysis of Incorrect Options:** * **Agranulocytosis:** While some anticonvulsants (like Carbamazepine) are associated with blood dyscrasias, this is not a result of an interaction with OCPs. * **Phenytoin toxicity:** OCPs do not significantly inhibit phenytoin metabolism; therefore, toxicity is unlikely. In fact, if an enzyme *inhibitor* (like Cimetidine or Valproate) were added, phenytoin toxicity would be the concern. * **Thromboembolism:** While OCPs themselves increase the risk of thromboembolism, adding phenytoin decreases the concentration of OCPs, theoretically reducing this risk rather than increasing it. **NEET-PG High-Yield Pearls:** * **Mnemonic for Enzyme Inducers (GPRS Cell Phone):** **G**riseofulvin, **P**henytoin, **R**ifampicin, **S**moking, **C**arbamazepine, **P**henobarbitone. * **Clinical Action:** Patients on OCPs starting an enzyme inducer should be advised to use an alternative method of contraception (e.g., barrier methods or an IUD) or a higher dose of estrogen (though the latter is less reliable). * **Rifampicin** is the most potent inducer and a frequent culprit in "OCP failure" questions.

Q: Which of the following is a P-Glycoprotein inhibitor?

Ketoconazole. **Explanation:** **P-glycoprotein (P-gp)** is an ATP-dependent efflux transporter found in the gut lining, blood-brain barrier, and renal tubules. It pumps drugs out of cells, thereby limiting their absorption and increasing their elimination. **Why Ketoconazole is Correct:** **Ketoconazole** is a potent **inhibitor** of both CYP3A4 and P-glycoprotein. By inhibiting P-gp, it reduces the efflux of substrate drugs (like Digoxin or Cyclosporine), leading to increased plasma concentrations and potential toxicity. Most drugs that inhibit CYP3A4 also tend to inhibit P-gp. **Why the Other Options are Incorrect:** * **Rifampicin:** This is a classic **inducer** of P-gp. It increases the expression of the transporter, leading to decreased plasma levels of P-gp substrates. * **Phenytoin:** Similar to Rifampicin, Phenytoin is a broad-spectrum microsomal enzyme **inducer** and also induces P-gp. * **Griseofulvin:** This antifungal agent acts as an enzyme **inducer**. It does not inhibit P-gp. **High-Yield Clinical Pearls for NEET-PG:** * **Key P-gp Inhibitors:** Verapamil (most potent), Quinidine, Ketoconazole, Itraconazole, Erythromycin, and Amiodarone. * **Key P-gp Inducers:** Rifampicin, St. John’s Wort, Phenytoin, and Carbamazepine. * **Important Substrate:** **Digoxin** is the prototype P-gp substrate. Co-administration of Verapamil or Ketoconazole with Digoxin can lead to **Digoxin toxicity** due to inhibited renal clearance and increased intestinal absorption. * **Location:** P-gp is encoded by the **MDR1** (Multi-Drug Resistance) gene. Overexpression of this gene in cancer cells is a major cause of resistance to chemotherapy (e.g., Vincristine, Doxorubicin).

Question 1

Which medication should be avoided in a patient with asthma who is currently on theophylline therapy, prior to dental extraction?

Accepted Answer

Ciprofloxacin

Answer

Lignocaine

Answer

Azithromycin

Answer

Metronidazole

Question 2

Which of the following is NOT an enzyme inhibitor?

Accepted Answer

Carbamazepine

Answer

Cimetidine

Answer

Valproate

Answer

Risperidone

Question 3

A patient with temporal lobe epilepsy (complex partial seizures) is experiencing recurrent seizures despite being on carbamazepine. Phenobarbital is added as a second drug, but the seizures increase in frequency. What is the probable reason for the apparent adverse effect of adding phenobarbital?

Accepted Answer

Decreased carbamazepine level

Answer

Intracerebral bleeding due to worsening bone marrow suppression

Answer

Decreased stability of CNS neuronal membranes

Answer

Hypokalemia

Question 4

The level of drug metabolizing enzymes in the liver determines how fast a drug is removed from the circulation. Therefore, it would be expected to find drug metabolizing enzymes:

Accepted Answer

Higher in smokers than in nonsmokers

Answer

Similar in smokers and nonsmokers

Answer

Lower in smokers than in nonsmokers

Answer

Simulated by malnutrition

Question 5

A 60-year-old male presents with muscle pain, fatigue, and dark urine. His medical history includes stable angina, and his current medications are atenolol, atorvastatin, and aspirin. Urine dipstick is positive for blood, but microscopy reveals no RBCs. Serum creatinine kinase is significantly elevated. Which of the following medication additions is most likely to have precipitated this patient's condition?

Accepted Answer

Erythromycin

Answer

Rifampin

Answer

Griseofulvin

Answer

Amikacin

Question 6

Which of the following drugs shows a disulfiram-like reaction with alcohol?

Accepted Answer

Griseofulvin

Answer

Amphotericin B

Answer

Terbinafine

Answer

Rifampicin

Question 7

Which of the following drugs should be avoided with warfarin?

Accepted Answer

NSAIDs like Ibuprofen

Answer

Antacids

Answer

Benzodiazepines

Answer

Codeine, dihydrocodeine, paracetamol

Question 8

Concurrent administration of tetracyclines is most likely to impair the action of which of the following drugs?

Accepted Answer

Penicillin

Answer

Lincomycin

Answer

Chloramphenicol

Answer

Erythromycin

Question 9

A woman who has been taking oral contraceptives for several years is diagnosed with epilepsy and started on phenytoin. Which of the following is the most likely consequence of adding phenytoin?

Accepted Answer

Reduced contraceptive efficacy

Answer

Agranulocytosis

Answer

Phenytoin toxicity

Answer

Thromboembolism

Question 10

Which of the following is a P-Glycoprotein inhibitor?

Accepted Answer

Ketoconazole

Answer

Rifampicin

Answer

Phenytoin

Answer

Griseofulvin

Drug Interactions — MCQs

Drug Interactions — MCQs

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